Fengying Zhang,
Pei Yu,
Wei Shen,
Ming Li and
Rongxing He*
Key Laboratory of Luminescence and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China. E-mail: herx@swu.edu.cn
First published on 17th July 2015
A series of “push–pull” sensitizers with modified conjugation bridges are designed and investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT), with the purpose of revealing the effect of different linker moieties on the performance of p-type dye-sensitized solar cells (DSSCs). Creatively, the electron-rich unit (thiophene) and the electron-deficient unit (pyrimidine) are studied as the linking groups in p-type sensitizers from a comparative perspective, two special bridge-sites and the lengths of conjugation bridges are also taken into account. Calculations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) indicate that there is efficient hole injection and dye regeneration for all the sensitizers. Importantly, the influence of the number of thiophene and pyrimidine moieties is seen mainly on the long-wavelength region and the short-wavelength region, respectively. According to the charge transfer properties and the driving forces of hole injection, dye regeneration and charge recombination (ΔGinj, ΔGreg and ΔGCR, respectively), the increased length of the thiophene-based bridge close to the carboxyl group has a positive impact on the device performance. Likewise, for the pyrimidine-based bridges, it is probably the increased conjugation length between the donor and acceptor that significantly improves the device efficiency. Our intensive analysis on the π-bridges provides assistance for designing more efficient p-type photosensitizers, which contributes to the rational design for tandem DSSCs.
Since the first self-operating p-type DSSC was prepared using erythrosine by Lindquist and coworkers,13 plenty of potential sensitizers, including perylene, coumarin, porphyrin and their derivatives, have been explored in succession.14–16 Upon the discovery of a new type of sensitizer with the “push–pull” configuration, 4-(bis-{4-[5-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl-amino)-benzoic acid (P1), the PCE reached 0.15% which is significantly higher than before.17–19 Currently, with the application of abundant modification measures for improvement, a photocathode efficiency of 1.3% was gained by Spiccia and co-workers in p-type DSSCs.20
Differing from the electron photoinjection into the conduction band of anode semiconductors such as TiO2 in n-type DSSCs, for p-type DSSCs, the hole is injected into the inorganic semiconductor (NiO) from excited sensitizers followed by electron transferring to the redox couple in the electrolyte.8,21–23 Plenty of studies have shown that the rapid back speed of the photoinjected hole from the p-type semiconductor to the reduced dye cuts down the separation efficiency of the charges, so as to limit the incident photon-to-current conversion efficiencies (IPCEs) and the resultant photocurrent.24,25 One of the effective methods to improve the efficiency of the charge separation is to increase the distance of the charge transfer by employing longer bridge groups between the electron-withdrawing group (acceptor) and the anchoring group when the anchoring group attaches to the electron-donating group (donor).26 Based on the typical “push–pull” sensitizer P1, Wu and coworkers27 shed light on the effect of different π-bridges, indicating that 3,4-ethylenedioxythiophene was superior to thienyl and phenyl in terms of IPCE. Li and co-workers26,28 synthesized and discussed the homologous sensitizer T1, in which an additional thiophene unit was inserted between the triphenylamine and carboxylic groups. They concluded that the performance of the device was improved significantly when the length of the bridge ligand near the anchoring group increases, while the increased conjugation bridge between the donor and acceptor had a negative effect on the device efficiency.
To further comprehend and demonstrate the effect of conjugation bridges of “push–pull” dyes on the device performance, a series of p-type sensitizers with modified conjugation bridges are designed and calculated. Herein, our primary interest in two kinds of bridges, one near the anchoring group (D-bridge) and another lying between the donor and acceptor (A-bridge), is to explore their effect on battery performance by employing electron-deficient pyrimidine and electron-rich thiophene, respectively. What’s more, the influence of their lengths is also taken into consideration. We believe that the better understanding of π-bridges, including the different electron-induced effects, position and length, could provide guidance for the further study of p-type sensitizers with higher PCEs.
![]() | (1) |
As one of the important parameters in the theoretical study for DSSCs, Jsc can be evaluated by the following integral equation:7,30,31
![]() | (2) |
LHE = 1 − 10−f | (3) |
ΔGinj = e[EVB(NiO) − (E0–0 + Ered(dye))] | (4) |
ΔGreg = e[E(I2/I3−) − Ered(dye)] | (5) |
Voc, the other determining factor for evaluating the performance of DSSCs, can be expressed as follows:8
Voc = E(I−/I3−) − EF(NiO) | (6) |
Additionally, the process of unfavorable charge recombination can be evaluated by its corresponding free energies ΔGCR, which can be expressed as:8,32
ΔGCR = e[Ered(dye) − EVB(NiO)] | (7) |
The reorganization energy of the S1 excited state Ereorg(S1) is determined by the difference of ES1(Q0) and ES1(Q1):33,34
Ereorg(S1) = ES1(Q0) − ES1(Q1) | (8) |
E0–0 = Eλmax − Ereorg(S1) | (9) |
To measure the nature of the photoinduced electron–hole separation in depth, related electron densities of sensitizers are calculated using the code Multwfn 2.5.36 Correspondingly, the distance of charge transfer and the fraction of charge exchange are calculated with DctViaCube.37
Generally, the electronic excitation of dyes as well as their transition characters are closely correlated with their FMOs:43,44 the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). As presented in Fig. 2, all the dyes show high LUMO energy levels for efficient electron transfer to the electrolyte (E(I2/I3−) = −4.15 eV),26 and sufficiently low HOMO energy levels for efficient hole injection into the semiconductor NiO (E(NiO) = −5.04 eV).26 Regarding the change of bridge length, calculations suggest that the energies of the HOMO and LUMO become higher slightly as the lengths of the bridges increase. On the one hand, thiophene-based bridges impact mainly on the HOMO energy level, and pyrimidine-based bridges near the acceptor have a great influence on both the HOMO and LUMO energy levels. However, on the other hand, energies of the HOMO and LUMO are almost unchanged when the D-bridge is a pyrimidine unit. Moreover, it can be speculated from the different energies between the HOMO and LUMO that the decreased band gaps realized in longer bridges are conducive to the red shift of the spectra.
![]() | ||
Fig. 2 Schematic energy levels of the sensitizers, together with the valence band of NiO and I2/I3− redox level. All calculations are obtained at the PBE0/6-31G(d) level. |
To the best of our knowledge, for the ideal sensitizers which have fast hole injection from the excited state of the dye into the valence band of NiO, the HOMOs of the dyes should be predominantly delocalized over the anchoring groups and their surroundings, and the LUMO distributions of the dyes should extend onto the acceptor groups and keep away from the semiconductor with the purpose of reducing hole recombination.26 All the distributions of HOMOs in our research systems are significantly located on the donor groups and their adjacent bridge-rings. As for the LUMOs, they are distributed mainly on the acceptor groups and their surroundings. Notably, with the lengthening of π-bridges, the distributions of LUMOs are increasingly far away from TPA, which is probably beneficial to the superior hole injection.
![]() | ||
Fig. 3 Simulated absorption spectra obtained at the BMK/6-31G(d) level for all the dyes, NiO–dyes and (NiO)9–dyes in acetonitrile solution with PBE0/6-31G(d) geometries. |
It is noted from Fig. 3 that the absorption spectra of all the dyes are scarcely affected whether or not the sensitizers are linked with the semiconductor NiO. For the two systems in which the sensitizers are linked with nickel oxide (NiO–dyes and (NiO)9–dyes, their optimized structures are presented in Fig. S1†), the spectral features can be simulated commendably, and their calculated spectra agree better with that of the isolated dyes. Based on further analysis of the spectra, the maximum wavelength of spectral absorption in the Q band appears red-shifted obviously as the lengths of the thiophene-based bridges directly linked with acceptor groups increase, from 498 nm (T-D-1S-A-1S), 527 nm (T-D-1S-A-2S) to 543 nm (T-D-1S-A-3S). In addition, the absorption strength also greatly improves, and the maximal oscillator strength of 3.3 is gained in T-D-1S-A-3S (from Table 1). Pyrimidine-based bridges close to acceptor groups have a significant influence on the B band. The absorption at 300–400 nm is enhanced dramatically as the lengths of the pyrimidine-based A-bridges increase. D-bridges, regardless of thiophene units or pyrimidine units, have little effect on the absorption spectra. With respect to the different π-bridges (thiophene-based bridges and pyrimidine-based bridges), the comparison of their absorption spectra implies that dye molecules with thiophene units as the conjugation bridges have a better ability to capture light.
Scheme | λmax (exp) | Ereorg(S1) | f | LHE | μT |
---|---|---|---|---|---|
a Data in parentheses are experimental values. | |||||
T-D-1S-A-1S | 498/344 (488/369) | 0.13 | 1.85/0.73 | 0.9859/0.8138 | 30.28 |
T-D-1S-A-2S | 527/372 | 0.15 | 1.91/0.61 | 0.9877/0.7545 | 33.03 |
T-D-1S-A-3S | 543/387 | 0.18 | 3.30/0.84 | 0.9995/0.8555 | 58.97 |
T-D-2S-A-1S | 504/385 | 0.12 | 1.77/0.91 | 0.9830/0.8770 | 29.34 |
T-D-3S-A-1S | 508/415 | 0.10 | 1.71/1.01 | 0.9805/0.9023 | 28.70 |
T-D-1S-A-1N | 497/326 | 0.07 | 1.61/0.93 | 0.9755/0.8825 | 26.41 |
T-D-1S-A-2N | 497/351 | 0.08 | 1.48/1.62 | 0.9669/0.9760 | 24.26 |
T-D-1S-A-3N | 482/379 | 0.10 | 1.30/1.50 | 0.9499/0.9684 | 20.70 |
T-D-1N-A-1S | 490/341 | 0.14 | 1.86/0.70 | 0.9862/0.8005 | 29.99 |
T-D-2N-A-1S | 493/342 | 0.14 | 1.85/0.72 | 0.9859/0.8095 | 30.03 |
T-D-3N-A-1S | 494/313 | 0.14 | 1.85/0.83 | 0.9859/0.8521 | 30.07 |
To clarify the light absorption properties of the dyes visually and accurately, some key parameters are gathered in Table 1. It is well known that f reflects the light-harvesting efficiency (LHE) to a certain extent owing to the limited dye loading on the semiconductor film.18 As expressed in eqn (3), the stronger the f, the larger the LHE. According to the analysis of the Q band, the largest LHE of 0.9995 achieved in T-D-1S-A-3S is not prominent in comparison to the 0.9859 of T-D-1S-A-1S. In contrast, the difference of 0.35 in f leads to the change in LHE from 0.9859 (T-D-1S-A-1S) to 0.9499 (T-D-1S-A-3N). In summary, the LHE has no significant change for dyes in the Q band. With respect to the B band, the increase of oscillator strength to 1.62 is realized in T-D-1S-A-2N in comparison with 0.73 (T-D-1S-A-1S). Correspondingly, its LHE is improved to 0.976 compared with 0.8138 (T-D-1S-A-1S), which is conducive to the capture of sunlight in the short wavelength region.
As for the transition dipole moment from states S0 to S1 (μT), it is another way to confirm the ability of light-harvesting and electron-transition. As shown in Table 1, μT is enhanced in the order of T-D-1S-A-1S (30.28) < T-D-1S-A-2S (33.03) < T-D-1S-A-3S (58.97), T-D-1N-A-1S (29.99) < T-D-2N-A-1S (30.03) < T-D-3N-A-1S (30.07). This sequence shows that μT is dramatically increased with thiophene-based A-bridge lengthening, but for pyrimidine-based D-bridges, μT shows little change. Inversely, it decreases as T-D-1S-A-1S (30.28) > T-D-2S-A-1S (29.34) > T-D-3S-A-1S (28.70), T-D-1S-A-1N (26.41) > T-D-1S-A-2N (24.26) > T-D-1S-A-3N (20.70). This order reveals the conclusion that the longer the thiophene-based D-bridges, as well as pyrimidine-based A-bridges, the smaller the μT.
The reorganization energy of the S1 excited state Ereorg(S1) increases with the lengthening of A-bridge, exactly as T-D-1S-A-1S (0.13) < T-D-1S-A-2S (0.15) < T-D-1S-A-3S (0.18), T-D-1S-A-1N (0.07) < T-D-1S-A-2N (0.08) < T-D-1S-A-3N (0.10). But for the change of thiophene-based D-bridges, the smaller Ereorg(S1) is realized in T-D-3S-A-1S (0.10). An identical Ereorg(S1) (0.14) appears in the sensitizers with pyrimidine units as bridges.
a Electron densities move from the green area to the purple area. | |||
---|---|---|---|
(a) | ![]() |
![]() |
![]() |
(b) | ![]() |
![]() |
![]() |
(c) | ![]() |
![]() |
![]() |
(d) | ![]() |
![]() |
![]() |
From Table 2, the electron density depletion (green) localizes largely on the anchoring groups, conjugation spacers and donor parts, whereas the electron density increment (purple) mainly localizes on the acceptor segments, which contributes to the electron moving from the green area to the purple area. Additionally, the zones of the overlaps for the electron density depletion and increment occur in the A-bridges and their surroundings. Interestingly, the length of the electron transfer reflects the overlap between the regions of density depletion and increment to some degree. The longer the charge transfer distance, the fewer overlaps for the electron density; accordingly, it leads to a good charge separation. For the situation of (a) in Table 2, when the conjugation moieties in the A-bridges are two thiophenes, the Ω is only 0.1934, and the corresponding L reaches a length of 6.219. However, when the number of thiophenes is up to 3, the electron transfer distance reduces to 3.780, and the Ω becomes 0.3601. The results indicate that the longer thiophene-based bridges between the donor and acceptor do not correspond to better charge separation. There is probably a nonlinear change of charge distributions as the number of thiophenes increases, and the increased number of thiophene units in the A-bridges does not promote charge separation.47,48 For (c), good charge separation is realized in different pyrimidine-based A-bridges, complying with the sequence of T-D-1S-A-1N (L = 4.515, Δe = 1.1681, Ω = 0.2726) < T-D-1S-A-2N (L = 6.261, Δe = 1.2914, Ω = 0.1264) < T-D-1S-A-3N (L = 8.023, Δe = 1.3276, Ω = 0.0522). This tendency implies that charge separation is enhanced significantly by lengthening the pyrimidine units in the A-bridges. Furthermore, changes in the D-bridges suggest that the electron transfer distance and the fraction of electron exchange can be improved dramatically for thiophene-based bridges, but are almost equal for pyrimidine-based bridges.
What’s more, for NiO–dyes and (NiO)9–dyes presented in Table S4,† it can be clearly seen that there is similar behaviour in the charge transfer between dyes linked with nickel oxide and the isolated dyes. In the (NiO)9–dyes, three molecules ((NiO)9-T-D-1S-A-1S, (NiO)9-T-D-2S-A-1S and (NiO)9-T-D-2N-A-1S) have a special way of electron transfer. Their electron density depletion localizes partly on the nickel oxide, which indicates that their electron densities move from (NiO)9 to the acceptor area.
The LHE has been evaluated by f in the section on optical properties. It is well established that the absorption spectra in the Q band are affected primarily by the electron-rich thiophene units, especially when they are located in the A-bridges. Conversely, the electronic-deficient pyrimidine groups exert a great influence on the absorption spectra in the B band, particularly when they are taken as the A-bridges. In addition, redox potentials of all the dyes are also calculated and analyzed, and the results are collected in Table 3. The electron injection driving force ΔGinj, a key parameter associated with electron injection efficiency Φinj, is calculated on the basis of eqn (4). It is easily found that ΔGinj is diminished a little as the lengths of the thiophene-based A-bridges increase. For the other situation, when the number of thiophene and pyrimidine moieties near the COOH group increases, the change of ΔGinj is negligible. The −ΔGinj is increased in the order 0.76 (T-D-1S-A-1N) < 0.88 (T-D-1S-A-2N) < 0.98 (T-D-1S-A-3N) as the number of pyrimidine units in A-bridges increases. Subsequently, the key regeneration process in DSSC devices has also been evaluated roughly through ΔGreg, according to eqn (5). Correlated data of ΔGreg in Table 3 elucidate that sensitizers (T-D-2S-A-1S and T-D-3S-A-1S) possess the greatest driving force of regeneration, 0.94 eV, compared with other dyes. Actually, previous literatures49,50 have revealed that both electron injection and dye regeneration can occur efficiently when the driving force is larger than 0.2 eV. As a whole, all the sensitizers have effective injection and regeneration.
Scheme | E0–0 | Eox(dye) | Ered(dye) | ΔGinj | ΔGreg | ΔGCR |
---|---|---|---|---|---|---|
T-D-1S-A-1S | 2.36 | 5.21 | 3.22 | −0.54 | −0.93 | 1.82 |
T-D-1S-A-2S | 2.20 | 5.04 | 3.29 | −0.45 | −0.86 | 1.75 |
T-D-1S-A-3S | 2.11 | 4.91 | 3.33 | −0.40 | −0.82 | 1.71 |
T-D-2S-A-1S | 2.34 | 5.10 | 3.21 | −0.51 | −0.94 | 1.83 |
T-D-3S-A-1S | 2.34 | 5.03 | 3.21 | −0.51 | −0.94 | 1.83 |
T-D-1S-A-1N | 2.42 | 5.28 | 3.38 | −0.76 | −0.77 | 1.66 |
T-D-1S-A-2N | 2.41 | 5.24 | 3.51 | −0.88 | −0.64 | 1.53 |
T-D-1S-A-3N | 2.47 | 5.20 | 3.55 | −0.98 | −0.60 | 1.49 |
T-D-1N-A-1S | 2.39 | 5.28 | 3.28 | −0.63 | −0.87 | 1.76 |
T-D-2N-A-1S | 2.38 | 5.26 | 3.22 | −0.56 | −0.93 | 1.82 |
T-D-3N-A-1S | 2.37 | 5.26 | 3.26 | −0.59 | −0.89 | 1.78 |
Additionally, based on eqn (7), the process of unfavorable charge recombination is also discussed and analyzed. From Table 3, it is not difficult to find that all the ΔGCR for dyes are positive, meaning that hindering the charge recombination is favored. For T-D-1S-A-1S, a great ΔGCR of 1.82 is achieved, and as the number of thiophene moieties in the D-bridge increases, the ΔGCR of 1.83 appears in T-D-2S-A-1S and T-D-3S-A-1S. With respect to the increase in lengths for A-bridges, including thiophene-based bridges and pyrimidine-based bridges, the ΔGCR complies with the sequence of 1.82 (T-D-1S-A-1S) > 1.75 (T-D-1S-A-2S) > 1.71 (T-D-1S-A-3S), 1.66 (T-D-1S-A-1N) > 1.53 (T-D-1S-A-2N) > 1.49 (T-D-1S-A-3N). In summary, inserting thiophene groups into the D-bridge can adequately modify the sensitizers’ energy levels and lead to lower charge recombination. According to the above analysis of important performance parameters, corresponding tendencies are presented intuitively in Fig. S2.†
Fundamentally and practically, the critical parameters (ΔGinj, ΔGreg and ΔGCR) are closely associated with the performance of p-type DSSCs, particularly for the Jsc and charge recombination. Related analysis indicates that increased lengths of thiophene units in the D-bridges have little influence on ΔGinj, ΔGreg and ΔGCR, but for the pyrimidine units in the A-bridges, they exhibit a favorable influence on the charge injection and recombination. Combined with previous research, we confirmed again that the increased lengths in thiophene-based bridges near the carboxyl group have a positive effect on the device performance. Similarly, for the pyrimidine-based bridges, it can be speculated that the increased conjugation lengths in the A-bridges could significantly improve the device efficiency. In addition, when comparing thiophene units with pyrimidine units, it is probable that pyrimidine units have a better contribution to the device efficiency, relatively, in our designed sensitizers. These results provide much guidance for the future design of p-type “push–pull” sensitizers with higher PCEs.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09263a |
This journal is © The Royal Society of Chemistry 2015 |